[time-nuts] Possibly off topic - Jitter on Ethernet over power adapters

James Harrison james at talkunafraid.co.uk
Sun Feb 10 14:35:19 EST 2013

Hash: SHA1

My gut feeling would be that overall noise/power/length of run etc is
going to be a significant factor, too - ie, longer runs or noisier
power environments will have an impact.

As with all things sensitive, it's best to isolate things - I have yet
to see any ethernet over powerline installations that couldn't be done
with 30m of category 5 cable and 10 minutes with a drill (or lots of
internal cable pins). Given that, my instinct would be to replace the
ethernet over power boxes with real cables. Another upshot: You'll
make any radio hams nearby very happy.


On 10/02/2013 19:00, David wrote:
> The poster is asking about ethernet over power line and not power
> over ethernet.  As you point out, the later should have zero effect
> on ethernet latency.
> There are several ethernet over power standards.  Latency will
> include a bridge in each adapter, the effects of a noisy
> uncontrolled AC power line when ARQ (automatic repeat query) is
> used, and the TDMA or CSMA media access control system.
> I suspect varying power line conditions will have the largest
> effect because any jitter from the media access control system will
> be multiplied by ARQ.
> On Sun, 10 Feb 2013 08:14:38 -0800, Chris Albertson 
> <albertson.chris at gmail.com> wrote:
>> THose power over Ethernet devices work with analog signals and
>> don't evn look at the data packets.   All they do is place a DC
>> bias on the twisted pair.    Ethernet is always transformer
>> coupled so your routers, switches and computers never see DC.
>> What is your NTP server using for a reference clock?  I'd suspect
>> that is the problem.   If the reference is an Internet pool
>> server than a few mS is about what you should expect.   If using
>> GPS then look to see if you have a good signal from enough
>> satellites.
>> But those POE boxes don't mess with the data packets, or at least
>> the are not designed to do that.   If one is broken it could be
>> adding noise to the line.  Broken hardware can do "anything".
>> On Sun, Feb 10, 2013 at 3:18 AM, Rob Kimberley 
>> <robkimberley at btinternet.com> wrote:
>>> I'm not sure if this is the best place to ask the question, but
>>> does anyone have experience of using Ethernet over power line
>>> adapters? I have an outside office, and my router is in the
>>> house plugged into the phone master socket. I have used two
>>> Ethernet over power adapters, one at the router and one in the
>>> office here to get internet access. The output of the adapter 
>>> then goes to a multi-port hub to give me Ethernet to all my
>>> office devices including two Meinberg NTP servers.
>>> I've noticed large jitter readings on Meinberg's NTP monitor
>>> program.  Can be as low as 2ms, but much higher (50mS +), and
>>> at this point NTP goes haywire.
>>> Not sure if it is the physical set up or something else.
>>> Any comments appreciated.
>>> Thanks.
>>> Rob
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